Plastic lenses are more lightweight, resistant to impact, and easily dyeable than conventional glass lenses. Due to these advantages, plastic lenses have recently been applied to most spectacle lenses. Particularly, plastic lenses typified by diethylene glycol bisallyi carbonate (CR-39) lenses have been applied as general lenses. These lenses with low chromatic aberration are useful in that they give a comfortable field of view, but have been required to have higher refractive index. Korean Patent Publication Nos. 1993-0006918 and 1992-0005708 propose thiourethane lenses manufactured by reacting polythiol compounds with polyisocyanate compounds. Further, Korean Patent Registration No. 10-0681218 proposes a thioepoxy plastic lens.
The thiourethane lenses have the advantages of high refractive index and impact strength but suffer from several problems such as weak surface and depressed center. The Abbe numbers of the thiourethane lenses tend to decrease markedly with increasing refractive index. The thioepoxy lens has the advantages of high refractive index and Abbe number but is likely to be brittle and is not readily dyed. Some attempts to solve the problems of thiourethane lenses and thioepoxy lenses have been proposed. For example, Korean Patent Registration No. 10-0417985 and Japanese Patent Publication No. Hei 11-352302 propose methods for manufacturing lenses by copolymerization of two resins having different properties, i.e. by copolymerization of thioepoxy, polythiol and polyisocyanate compounds.
However, some thioepoxy lenses including thioepoxy compounds as main monomers are still unstable in color. Korean Patent Registration No. 10-0681218 reveals that the purity of a thioepoxy compound converted from an epoxy compound, i.e. the content of by-products of the conversion process in the thioepoxy compound, is a factor affecting the color of lenses. Further, this patent discloses a polymerizable composition including a reduced amount of the by-products and a method for reducing the content of the by-products.